The oil now coating the Gulf of Mexico in reddish brown streaks has a long journey behind it. Tracing that journey would require diving 1,500 meters (5,000 feet) into the ocean, passing through a massive layer of mud and finally pounding through hard salt.

The oil originated more than four kilometers (two and a half miles) below the ocean floor, in rock layers that formed millions of years ago, during the Tertiary period. It’s scalding hot down there, a veritable journey into hell, but companies such as BP, Shell, ExxonMobil and Chevron are daring to make the trip more and more often these days. Flying over the site where the drilling rig Deepwater Horizon sank in late April reveals dozens more oil platforms projecting out of the water on the horizon, like toys bobbing in a bathtub. … More